fix: dashboard errors — Math.abs, API routing, fetchHistorical guard

Fixes three bugs in dashboard:

1. vbt.html: bare abs(dd) → Math.abs(dd) — fixes ReferenceError
2. server.py: Add /cv/ routing for Next.js quant dashboard
   - Mount _next static assets at /cv/_next (not /cv/ which eats API routes)
   - Add /cv/api/* routes for backtests/historical, detail, recalc, risk
   - Add /cv/ws WebSocket endpoints for live/paper metrics
   - Add /cv/ catchall for Next.js HTML pages
3. dashboard-next/src/lib/api.ts: add res.ok guard to fetchHistorical()
   — prevents SyntaxError when API returns HTML error pages
4. sim/maker.py: guard observe() against zero mid_price

dashboard-next/out/_next/static/41wlgrdKzoSUpkrWIQ9jn/_buildManifest.js

@@ -1,11 +0,0 @@
-self.__BUILD_MANIFEST = {
-  "__rewrites": {
-    "afterFiles": [],
-    "beforeFiles": [],
-    "fallback": []
-  },
-  "sortedPages": [
-    "/_app",
-    "/_error"
-  ]
-};self.__BUILD_MANIFEST_CB && self.__BUILD_MANIFEST_CB()

dashboard-next/out/_next/static/41wlgrdKzoSUpkrWIQ9jn/_clientMiddlewareManifest.js

@@ -1 +0,0 @@
-self.__MIDDLEWARE_MATCHERS = [];self.__MIDDLEWARE_MATCHERS_CB && self.__MIDDLEWARE_MATCHERS_CB()

dashboard-next/out/_next/static/41wlgrdKzoSUpkrWIQ9jn/_ssgManifest.js

@@ -1 +0,0 @@
-self.__SSG_MANIFEST=new Set([]);self.__SSG_MANIFEST_CB&&self.__SSG_MANIFEST_CB()

dashboard-next/out/_next/static/CxC9DIW0Ude54_bbbG52g/_buildManifest.js

@@ -1,11 +0,0 @@
-self.__BUILD_MANIFEST = {
-  "__rewrites": {
-    "afterFiles": [],
-    "beforeFiles": [],
-    "fallback": []
-  },
-  "sortedPages": [
-    "/_app",
-    "/_error"
-  ]
-};self.__BUILD_MANIFEST_CB && self.__BUILD_MANIFEST_CB()

dashboard-next/out/_next/static/CxC9DIW0Ude54_bbbG52g/_clientMiddlewareManifest.js

@@ -1 +0,0 @@
-self.__MIDDLEWARE_MATCHERS = [];self.__MIDDLEWARE_MATCHERS_CB && self.__MIDDLEWARE_MATCHERS_CB()

dashboard-next/out/_next/static/CxC9DIW0Ude54_bbbG52g/_ssgManifest.js

@@ -1 +0,0 @@
-self.__SSG_MANIFEST=new Set([]);self.__SSG_MANIFEST_CB&&self.__SSG_MANIFEST_CB()